CHIEF FUNCTIONS OF THE ANIMAL BODY. 25 



A piece of living muscle consists of fine transparent 

 tubes or fibres, each invested by a sheath or sarcolemma, 

 while the whole muscle is surrounded by connective tissue. 

 It usually runs from one part of the skeleton to another, 

 and is fastened to the skeleton by tendons or sinews. It 

 is stimulated by motor nerves, and is richly supplied with 

 blood. 



When a muscle contracts, usually under a stimulus 

 propagated along a motor nerve, there is of course a 

 change of shape — it becomes shorter and broader. The 

 source of the energy expended in work done is the 

 " chemical explosion " which occurs in the fibres, for the 

 oxygen stored up (intramolecularly) in the muscle enters 

 into rapid union with carbon compounds. Heat, C0 2 , and 

 water are produced as the result of this combustion, and 

 lactic acid is also formed as a by-product. Besides the 

 chemical change and the change of shape, there are also 

 changes of "electric potential" associated with each con- 

 traction. 



Digestion. — The energy expended in doing work or in 

 growth is balanced by the potential energy of the food 

 stuffs taken into the body. These consist of proteids, 

 carbohydrates, fats, water, and salts, in varying proportions 

 according to the diet of the animal. Oxygen may also be 

 regarded as forming part of the food. 



In some of the lower animals, such as sponges, the food 

 particles are directly engulfed by some of the cells with 

 which they come in contact. Within these cells they are 

 dissolved : this is known as intracellular digestion. In 

 most cases, however, the food is rendered soluble and 

 diffusible within the food canal, by the action of certain 

 ferments made by the cells which line the gut or form the 

 associated glands. The great peculiarity of these ferment- 

 ing substances is that a small quantity can act upon a large 

 mass of material without itself undergoing any apparent 

 change. But however digestion be effected, it means 

 making the food soluble and diffusible. In a higher 

 vertebrate there are many steps in the process. 



(a) The first ferment to affect the food, masticated by the teeth and 

 moistened by the saliva, is the ptyalin of the salivary juice, which 



